10.0–30.0 g portions of oven-dried samples were
placed into a flask (made specially for this purpose,
from Gorsuch [20]) and concentrated nitric acid (5 ml
of acid for 10.0 g of dried fruit) was added and heated,
with stirring, for 30 min. Then, a mixture of nitric
acid–hydrogen peroxide (1+1) (for 10.0 g of dried
sample, 20 ml of mixture) was added. The mixture
was digested, with stirring, until a clear digest was
obtained (approximately 4.0 h). Then, the clear digest
was diluted to 150 ml with distilled water. A blank digest
was carried out in the same way.